The movement of flexion of the knee occurs for the first 30 degrees according to a circular path around a fixed center during which the femoral condyle rolls on the tibial plateau. After this first phase, and then after 30 degrees the movement continues with a roto-translation phase characterized by the sliding ever more progressive of femoral condyle on the tibial plateau. Given the anatomical conformation of the condyle itself, this causes a progressive decrease of the distance between the instantaneous center of rotation and the articular surface. It is therefore evident there is a difference between the angle measured with a goniometric system and that really done from the knee, because in the phase of roto-translation the variation of the position of the center of rotation continuously changes spatial references of evaluation.
To make more explicit the above it is essential to consider a reference system X-Y in which the axis X coincides with the longitudinal axis tended limb that is, when the longitudinal axis of the thigh is in line with the longitudinal axis of the leg itself. The axis X intersects orthogonally Y and anatomically intersects horizontally the femoral condyle (FIG. 1). The initial center of rotation of the knee, and so, coincides with the origin of the Cartesian reference system X, Y.
For the evaluation of the movement amplitude of the knee it is therefore essential to provide a system which takes into account the displacement of the initial center of rotation during the roto-translational phase.
To describe the gradual shift of the axis of rotation of the knee, established that Ra is the first radius of rotation that lies on the axis Y, the trajectory of flexion-extension of the leg on the thigh is analytically defined:
for α<30 degrees knee motion is described as a rigid system that rotates around a fixed center: the trajectory performed by the point P has as equation that of a circle X2+Y2=Ra2.
for 30 degrees≦α<135 degrees, the center of rotation moves towards the articular surface of a quantity equal to Δx.
The new coordinates of point P become:X1=X+Δx Y1=Y+Δy 
The equation of the center of rotation of the knee appears to be:X12+Y12=Rb2 
where Rb is the real rotation radius that changes with the change of α, with Rb<Ra
When α is the angle between the axis X and the rotation radius, the values of X, X1, Y and Y1 are so obtained:X=Rasen αY=Ra cos αX1=Rbsen αY1=Rb cos α
So for a given value of α between 30 and 135 degrees the position of the instantaneous center of rotation is calculated:Δx=X1−X=(Ra−Rb)sen α
From 30 degrees of flexion onwards (135 degrees) the center of rotation, initially placed in the origin of the reference system, vertically slides towards the articular surface along the axis X by an amount equal to Δx. The radius Ra remains unchanged in its length and drag the point P on a spiral path falling towards the center.
This determines that the distance between P and the origin of the reference system X-Y is reduced (Rb) (FIG. 2). After the 30 degrees, then, Rb whose first end is always centered in the origin of the reference system X, Y to the changing of α will assume different angular values of those of Ra which remains the actual radius of rotation of the knee.
From the above, it appears that in the roto-translational movement the point P placed at one end of the radius Ra follows a curve with a spiral path falling towards the center, while the first end of the radius Ra slides along the axis X. This implies that the scale for the evaluation of the real angle of the knee with reference to the radius Ra must be built according to the different movement of the two ends of radius Ra itself (FIGS. 3-4).
Several patents have faced the problem of roto-translational motion of the knee, but the applications were generally limited to application of mechanical devices to apply to the injured knee, restricting movement to an amplitude considered sufficient to second a “physiological” flexion of about 130 degrees, as necessary amplitude for a total recovery of functionality. Among these we cite the EP 0 361 405, the WO 84/03433, the WO 92/15264 and the WO 97/38759.
The knee joint described in the European Patent No. 0 361 405 is based on the physiological concept whereby the flexing of the knee consists in the fore movement of the femoral condyles with reference to the tibia condyles, followed by a sheer rotation between the condyles of the above mentioned bones. This joint features three plates, of which the two outer ones feature coaxial holes, while the inner one features two openings where a pair of pins that fit through the above mentioned holes in the outer plates are lodged and guided. One of the openings is small and extends transversally across the longitudinal axis of the tibia and femur, while the other opening is large and is shaped like a circular segment with one end growing wider towards the top.
The first opening, the transversal slot, has the function of reproducing the first fore movement of the femur with reference to the tibia, while the second opening serves the purpose of guiding the subsequent rotation movement.
The upper end of the circular opening is placed on the extension of the longitudinal axis of the arm of the central plate which passes through the centre of the pin lodged in the linear opening, precisely when the pin is halfway through the stroke performed by the pin inside this opening. The centre of the circular segment that constitutes the circular opening consists in the centre of the pin lodged in the linear opening when the pin itself is at the end of the said opening, which is the one farthest from the circular opening.
When the leg flexes, in the first 25° the pin lodged in the circular opening compels the pin in the linear opening to move from its starting position (closer to the circular opening) to its final position (at the end of the linear opening that lies farthest from the circular opening).
As the distance between the centres of the pins is equal to the radius of the circular opening, in this first part of the movement performed by the pin lodged in the linear opening, the pin lodged in the circular opening performs a small vertical movement within and outside the widened part that constitutes the upper part of the circular opening. In this first phase of the flexing movement the two outer plates slide forward with respect to the inner plate (traction or pulling apart phase of the two plates).
Subsequently the two outer plates rotate onto the inner one as the pin at the end of stroke position in the linear opening acts as a fulcrum and compels the pin lodged in the circular opening to move (sheer rolling phase).
The knee tutor joint described in the International Patent Application WO 84/03433 consists of five plates. The two outer ones, connected to the supporting elements of the lower leg, each feature a hole and a linear opening. The two intermediate plates also feature a hole and a linear opening each, but in the opposite position compared to the holes and openings of the outer plates.
The central plate, which is connected to the supporting elements of the upper leg, features a central hole and a bending opening which extends completely within the plate itself and which simulates the crosswise course of a flexing point on a given patient's knee.
The plates are locked onto one another and they can each pivot around one another and around the central shaft. This shaft extends through the linear openings in the outer plates and the central holes of the intermediate and central plates.
A pin passes through the peripheral holes of the two outer plates, the linear openings in the intermediate plates and the bending opening of the central plate. The central shaft and the pin lock the plates onto one another in such a way that the restricted movement of the pin inside the bending opening limits the movements of the supporting elements of the lower leg with respect to those of the upper leg: hence, the flexing and extension of the patient's lower leg is limited. The bending opening lodges some flexible pistons, which act as springs. These can move and are fastened to the ends of the bending opening in order to limit the movement of the pin and, consequently, the width of the flexing movement. These flexible pistons are locked by two threaded bolts next to each of which lies an indicator that moves longitudinally to the pistons themselves. The function of the indicators is to indicate the degrees of movement allowed in the flexing and extension movements: from 0° to 140°.
If one postulates that the central plate stays still, in this joint the intermediate plates rotate and move with respect to the central one. The outer plates rotate along with the intermediate ones but they move to the side of the latter plates along the axis in the direction of the linear openings in the intermediate and outer plates. In this system the outer plates move (rotate and slide) with respect to the central plate.
The central shaft and the pin that passes through the peripheral holes of the outer plates also change distance between them in the flexing movement.
However, there is no prior identification of the centre of the knee; this centre ought to be aligned with the central shaft. Furthermore, the possibility of personalising the bending opening is not described other than by referring to the extreme limits imposed on the movements.
The device described in the US2006/247565 proposes a pure circular motion with a goniometric system for setting the movement range. The joint is completed by two block systems for the extension and the flexion that can be located to several angles-shots. Such mechanical elements rotate around to a fixed center thus proposing only a partial reproduction of the physiological motion of the knee.
There are few patents which deals with a movement extends to larger degrees of freedom, even up to 150 and 160 degrees that is done in terms of prevention or that are able to support the knee of athletes in specific sports which occasionally undergo to particularly high loads that need, to be absorbed, a hyper-flexion of the knee. Among these we cite U.S. Pat. No. 4,643,176/A1.
Are further known US 2001/056012 and US 2005/148916.
Instead, we do not know of any patents that describe a measuring device of the actual angle reached by the knee in its phase of flexion. Today this evaluation occurs with the use of the traditional goniometer. The longitudinal axis of an arm is superimposed to the longitudinal axis of the thigh; the longitudinal axis of the other arm is superimposed to the longitudinal axis of the tibia. The center of rotation of the goniometer must be superimposed on the initial center of rotation of the knee in a way that the orthogonal axis to the rotation surfaces of the goniometer arms is coaxial with the transverse intercondylar rotation axis “c” of the knee itself
But during the evaluation phase, after 30 degrees, the goniometer not proposing sliding between its two arms, that is what normally happens in the knee instead, is forced to let slide an arm on an anatomic segment. In fact, if we keep the instrument fixed to the thigh the goniometric distal arm moves relative to the leg, if we keep the instrument fixed to the leg the goniometric proximal arm moves relative to the thigh. This continual movement of the goniometer is not able to ensure that all the contact points remain constant from the beginning to the end of evaluation. This drawback let dropped the main feature to which must correspond an evaluation device, which is the possibility of the measurement repeatability.
And, always for the same reason, it is evident that also the notorious knee braces are not able to maintain during flexion the overlap of the longitudinal axis of their tibial arms with the longitudinal axis of the leg itself.